Thank you Christopher for your reply. It helped a lot...
Now, after sky subtraction and using right formula the magnitudes are correctly estimated for my list of galaxies.

Cheers,
Artur.
 

I have a couple of questions.
Will be grateful to have answers.

When downloading fits image from Image Cutout:
1) Are the fits images calibrated and sky-subtracted in each band?

2) Can I measure my own magnitudes on downloaded image by the...